As is widely known, a golf ball may be formed by using a forming mold that can be divided into two parts, the mold, having a spherical cavity therein, being formed with a large number of protrusions for forming dimples (dimple-forming protrusions) on the wall surface of the cavity, and having a parting line substantially at the position of the equator of the cavity.
In the case of using such a mold to obtain a golf ball that has a core covered by a cover layer formed by injection molding on the surface thereof, the core is disposed at the center of the two molds and is supported by a supporting pin, and the cover layer is formed by covering material by injection molding in the space between the inner surface of the mold and the core surface.
In this type of injection molding, after the covering material is injected and the golf ball is formed, when the supporting pin is pulled out, pin burrs occur on the surface of the golf ball. Additionally, the hardened cover material within the gate for injecting covering material (resin material) of the ball remains in a projecting state, forming protrusions, and in addition, and fine burrs are also produced at the parting line position of mold.
Because these burrs are a cause of greatly degraded aerodynamic characteristics of the golf ball, it is necessary to remove them by grinding after molding. However, in the grinding process to remove these burrs, the dimples are also ground, the size thereof being reduced, and this leads to a problem in that the surface area occupied by the dimples is reduced from the value at the time of design. Also, deformation of the dimples due to grinding is sensitive to the strength of grinding, so that there were variations between golf balls.
The specification of the Japanese Patent Application Publication No. 60-142870 describes a method for maintaining the shape of the dimples while removing burrs, by making the area surrounding the dimples along the parting line have a specific shape. Also, Japanese Patent Application Publication No. 11-114092 describes a method of forming protrusions near the dimples on the equator to avoid abrasion of dimples due to grinding. Various methods have been proposed to avoid the abrasion of the dimples by grinding in the area adjacent to the parting line. However, in spherical grinding for the purpose of removing pin burrs, insufficient consideration has been given to maintaining the shape of dimples over the entire surface of the golf ball.